Common Flat Roof Problems and How to Prevent Them

For many buildings and homes, flat roofs offer a cost-effective (especially when compared to traditional pitched roofs), attractive, and easy-to-manage roofing system.

But like all roof structures, there are some common flat roof problems, like ponding water, leaks, and roof membrane shrinkage, that occur without the correct care and maintenance.

To help you look after and prolong the life of your new flat roof, we’ve discussed some of the most common problems and how to prevent them with expert advice.

Flat Roof Issues

Flat Roof Leaks

Leaks are one of the most common issues that flat roofs face. They can happen for many reasons, including ponding or standing water, damage to the flashing, or poor installation of the roofing material.

Often, flat roof covering leaks start small and turn into a nightmare problem over time. If you have a flat roof, watch out for damp, dark brown patches on your ceiling, sagging and dipping, and any low spots where water might pool. These are all warning signs, and if you can catch them early, you’ll hopefully avoid a big leak and other serious problems!

How to Prevent Flat Roof Leaks

• Check the flat roof at least twice a year and after major storms and any other harsh weather. Look for signs of bubbling, cracks, exposed seams, or soft spots that indicate underlying moisture. This regular maintenance will help keep on top of any issues.
• Keep seams and flashing sealed with professional-grade waterproof coatings or tapes. Pay special attention to areas around HVAC units, vents, and drains.
• Use flexible, weather-resistant flashing installed correctly by a reputable roofing contractor to account for building movement and expansion. Inspect it annually for gaps or wear.
• Address even minor leaks or membrane damage immediately. The longer water infiltrates, the more extensive (and expensive) the repairs.
• Keep the flat roof clean and free from branches, leaves, and other debris, and limit unnecessary foot traffic. If access is needed, use walkway pads or mats to reduce membrane wear.
• Keep an eye on the proper drainage on your flat roof to make sure it stays clear. If backed up, there can be a buildup of water.

Ponding Water on Flat Roofs

Ponding water is water that doesn’t drain 48 hours after rain. If left, it can create stress on the membrane and lead to structural damage to your flat roof.

It’s usually caused by bad drainage, an improper roof slope, or debris build up. While flat roofs are not perfectly flat (they’re built with a slight pitch for drainage), even a minor sag or blocked drain can cause water to pool in low spots.

How to Prevent Ponding Water

• Schedule regular inspections and cleanings of all roof drains, scuppers, and downspouts, especially in the autumn or after storms. A blocked drain is one of the quickest ways to cause ponding.
• Ensure your roof has an adequate slope or drainage system during construction or replacement. Tapered insulation can be installed under the membrane to help direct water away from problem areas.
• If certain areas repeatedly collect water, it may be worth installing extra drains or scuppers to improve water evacuation.
• Some flat roof coatings are designed to resist damage from standing water and UV exposure. These can protect vulnerable areas temporarily while a more permanent fix is planned.
• After heavy rain, inspect the roof to identify any pooling areas. Mark trouble spots and track how long water remains to determine if there’s a chronic drainage problem.
• If ponding is due to sagging or structural issues, consult a roofing or structural professional to assess and reinforce the roof deck before replacing the membrane.

Membrane Material Shrinkage

Over time, the membrane on a flat roof can shrink. This mainly happens to single-ply flat roofing systems like EPDM. It happens because the membrane can contract and pull tight, especially at the edges and the seams.

This tension can cause stress to the flashing, open seams, and cause cracks and tears. This then makes the flat roof vulnerable to leaks and wind uplift.

Membrane shrinking is a slowly developing issue, but there is no easy fix for this.

How to Prevent Membrane Shrinkage

• Choose membranes with resistance to shrinkage and weathering. Newer products like TPO and PVC may offer better stability in some climates than EPDM.
• Ensure the membrane is fully relaxed before being adhered. Installers should follow manufacturer guidelines to allow for thermal movement.
• Install additional mechanical fasteners or edge restraints at the roof perimeter to resist tension from shrinking membranes.
• Inspect around edges, corners, and penetrations where shrinkage is most likely to show up first. Look for lifted flashing, stressed seams, or curled edges.
• UV-reflective coatings can help reduce surface temperature and slow the aging process, particularly in hot climates.
• Don’t ignore minor separations. Sealing or patching them early prevents water infiltration and more extensive damage.

Flat Roof Cracking and Blistered Material

On a flat roof, cracks and blisters can be a physical sign of stress, and they are often a warning sign for other, more severe issues.

The blisters are raised bubbles caused by trapped moisture or air below the membrane. Cracks are due to age, harsh weather, and they compromise the waterproofing layer.

If you don’t repair blisters and cracks, they can evolve into membrane failure.

How to Prevent Cracking & Blistering

• The installer should make sure the roof deck is completely dry before installing the membrane. Trapped moisture is a major cause of blistering, especially in built-up and modified bitumen roofs.
• Reflective coatings can protect the membrane from solar damage and prolong its flexibility.
• Look for early signs of blistering, cracking, or alligatoring (a pattern of surface cracking) during biannual inspections. Early intervention is much cheaper than full repairs.
• Use walkway pads or mats in high-traffic areas to reduce stress and impact damage on the membrane.
• If possible, improve ventilation in the roofing assembly to minimise trapped heat and moisture.
• Blisters that pop or cracks that widen allow water in. Promptly seal and patch small imperfections with compatible materials before they worsen.

Flat Roof Organic Growth (Moss, Algae, Mold)

Flat roofs in damp, shady, and humid areas may be susceptible to the growth of moss, algae, and mold that thrive in areas where moisture lingers.

This is different from a green flat roof, as these are specially designed with drainage and protection layers between the plants and the flat roof. With this organic growth, it can trap moisture and degrade roofing materials.

How to Prevent Organic Growth on a Flat Roof

• Regularly remove leaves, dirt, and debris from the roof surface and gutters to eliminate organic matter that retains moisture.
• Overhanging branches not only drop debris but also create shaded, humid zones where moss and mold thrive.
• Make sure all water exits the roof quickly. This includes cleaning scuppers and downspouts, and fixing low spots that hold water.
• If mold appears due to condensation under the membrane, consider improving ventilation or using vapor barriers to control interior moisture migration.